Electric discharge device



pct. 27, 1936. ,A AB A-H' 2,059,037

' ELECTRIC DISCHARGE DEVICE Filed Jan. 6, 1953 I g 4 r 4 IIIIIIIIIIIIII lnventori Carn i' l A. Sabbah,

145% H is Attovneg.

Patented Oct. 27, 1936 UNITED STATES 2,059,037 v ELECTRIC DISCHARGE DEVICE Camil A. Sabbah, Scotia, NIL, assignorto General Electric Company, a corporation of New York Application January 6, 1933, Serial No. 650,511 3 Claims. (01. 250-275) My invention relates to electric discharge apparatus such as the mercury arc rectifier or inverter, particularly to the shield or control electrode devices or the like associatd with the anodes 5 of such apparatus, and has for its object the provision of an improved anode shield or control electrode device whereby the occurrence of cathode spots thereon is prevented while permitting the device to assume a desired potential.

In the mercury arc rectifier and like apparatus it is customary to provide metallic members interposed between the anodes and cathode, these members functioning as anode shields or arc guides, or as control electrodes or grids, or simull5 taneously as anode shields and control electrodes. The members may be so mounted as to surround the associated anode except for an opening permitting the arc discharge to pass, or may be mounted below or adjacent the anode in the path Difficulties have been encountered, however,

in the use of these metallic shield or grid members. In many instances, for example, steel shields have been found to be perforated and heavily corroded in operation by an arc discharge passing through the material of the shield instead of around its edge to the anode, this action of the discharge forming on the shield a cathode spot which, by reason of the condensation upon the.

associated anode of the metal vapor thereby formed, caused are back of this anode.

In order to avoid this arcing difi'iculty en countered in the use of steel or other metallic shield or grid members it has been proposed to form thesemembers of insulating material. 'But the use of this latter material for the purpose introduces other difficulties, since if'insulating material is used for anode shields or arc guides-difliculty is encountered in starting the arc discharge 4() in the rectifier, and if used for control electrodes or. grid members these control members cannot be given a definite required potential.

In accordance with my invention these difficulties are overcome, and the advantages of a metallic or conducting shield or grid member are retained at the same time that the disadvantages thereof are obviated, by the provision of a shield or grid member composed of a material through which the discharge currents causing formation 50;; of cathode spots and'consequent arcing in the metaltype. of shield or gridmember cannot flow, but "whose surface nevertheless is capable of assuming a definite potential. To attain this result I provide a shield or grid member char- 55, acterized by such resistivity that substantial flow of the arc discharge between anode and cathode.

of current cannot occur therethrough but having a surface which, while of high resistivity, is capable'of being charged to a desired potential.

This member is preferably constituted by a body or mass of insulating material covered with a thin film of-conductive material. A shield or grid member thus formed will have an equipotent'ial surface but will not permit an arcof appreciable intensity to discharge on any point of its surface since the current supplying such an arc is limited entirely to the current which can fiow in the surface layer. As this layer is of extremely high resistivity, being preferably a metallicfilm of molecular order of thickness, the surface current is of insuflicient magnitude to start or sustain a cathode spot.

In my application for United States Letters Patent, No. 1,855,154, I have disclosed such an equipotentialmember. In the aforesaid application this equipotential member is shown as an anode shield'of cylindrical form surrounding the anode and extending downwardly of the anode tip to a point in the evacuated container adjacent the lower wall thereof. The form of this member asdisc1osedin the aforesaid application is, therefore, essentially the same as that of'the usual anode shield or housing constituted by a steel tube in the upper portion of which the anode is mounted andwhich extends downwardly within the evacuated container.

The present application discloses an equipotential member of a different form, which I have found preferable in certain instances, particularly in'cas'e .theanode with which the equipotential member is associated is of the external type, or of the type similarin general to that shown for examplein Patent No. 1,677,689 to Robinson and Sabbah,-..or in Patent No. 1,752,204 to Sabbah, although it is to be understood that the equipotential member of the present invention is not limited to use with anodes of the abovementioned type.

Incarryihg my present invention into effect a'nfequipot ential member is provided comprising a large'body of material presenting a relatively largedeionizing surface in the arc path between cathode and anode, this member having an upper tubularpo'rtion adjacent active surfaces of the anode and'a preferably plate-like lower portion, of-relatively large diameter and of relatively extensive surface area, provided with openings for the passage of the arc, the openings being preferably formed at an angle with the plane of this lowerportion in order to increase the area of the deionizing surface of the equipotential member and also to prevent solid or liquid particles from being projected against the anode surface from the walls of the container and from causing a cathode spot.

In case the anode is of the hereinbefore mentioned external type and comprising, for example, a hollow, relatively long conductive element extending outwardly from or outside of the evacuated container, the aforesaid upper tubular portion of the equipotential member may extend adjacent the lower external surface only of the hollow anode, or may extend entirely within the anode and upwardly to a point adjacent the upper end thereof.

My invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 is a fragmentary cross-sectional view of an electric discharge device in which the present invention has been embodied, and Fig. 2 is a view illustrating a modification of the present invention.

In Fig. 1 is shown an electric discharge device I such as a mercury arc rectifier or inverter comprising an evacuated container or tank II, a mercury cathode I2, and a plurality of anodes I3 only one of which is shown in the figure. The mercury arc rectifier or inverter I0 is shown in the drawing as of the large iron tank type adapted to the transfer of relatively heavy currents, as contrasted with the current load in such discharge devices as mercury arc lamps. In the ,present embodiment of the invention the anode I3 is shown as being of the external type and comprising a hollow member or tube I4 of relatively great depth or length which may be fastened to a tubular mounting member I5 insulatingly connected to the container. For this purpose the mounting member I5 may be provided with a flange portion I6 molded in a suitable insulating member or plate I! preferably formed of a mixture of mica and lead borate. Insulating member I! may constitute the cover plate of a usual anode shield or housing I8 and may be secured theretoby suitable clamping means In the path of the arc discharge between cathode I2 and the active surfaces I9 of anode I3 is mounted a member including a mass or masses of material 2| sufficiently resistive to prevent substantial flow of current therethrough but having surfaces 22 sufficiently conductive to 0 .permit the charging thereof to a definite potential and at the sametime sufliciently resistive to prevent any substantial flow of current thereover.

In the present embodiment of the invention, in order to prevent an arc discharge from passing through the main mass. or body 2| of the member 20, this main mass or body is preferably composed of a suitable heat-resistant insulating material, such for example as fired soapstone. In order atthe same time to permit the surfaces 22 of the member 2|] to assume a desired potential, the insulating material of which this body or mass 2| is compos'ed'is coated with a thin, preferably metallic, conducting film 23 of high resistivity to prevent flow of current over these surfaces sufiicient to cause the formation of cathode spots on the member 20. To insure the desired conductivity for charging current and at the same time to insure the resistivity suflicient to prevent substantial current flow through the surface film, the film 23 is preferably of a molecular order of thickness only.

To facilitate manufacture and assembly, the equipotential member 20 is preferably formed in two parts. One of these parts, the upper, is mounted in the insulating member IT as by being threaded or molded therein and comprises a tube portion 24 coated on both inner and outer surfaces with the aforesaid thin film 23 and extending a substantial distance above the point of attachment to member I! and to a point adjacent the lower portion of anode shell or tube I4. The other part of member 20, the lower, is a preferably plate-like member 25 of relatively large diameter and substantial thickness and having a plurality of openings 26 formed therein for the passage of the are, these openings being preferably at an angle, as 45, with the plane of the member 25. In order that the plate member 25, having surfaces capable of being charged to a definite potential, may extend across substantially the entire arc discharge path, this member 25 is preferably of only slightly less diameter than that of the anode shield or housing I8, as shown in the figure. To connect the tube portion 24 and the plate-like portion 25 the upper part of member 20 comprises a preferably conical extension 21 fastened to the lower portion 25 in any suitable manner as by screws or pins 28.

It will be seen that the member 20, comprising the tubular portion 24 of substantial length and the plate-like portion 25 of large diameter and having a plurality of openings which are relatively long in the direction of the arc, provides an equipotential surface 22 of large area in the path of the arc between the cathode I2 and the active surfaces I9 of the anode tube or shell I3.

It will be seen, further, that since the portion 29 of the conductive film 23 which is on the outside surface of tube portion 24 is well shielded from the are between anode and cathode, an insulated lead for member 20 may easily be provided if required, passing out from the evacuated space, for the purpose, for example, of connecting the conductive layer 23 of equipotential member 20 to the anode I3 through a high external resistance, or to impress potentials on the member 20 from an outside source, or for other purposes.

In Fig. 2 an equipotential member 3|] similar in general to the equipotential member 20 of Fig. 1, i. e., having an upper tubular portion and a lower thick plate-like portion connected to the upper portion by a conical extension, is shown, associated with the anode I3. In the arrangement illustrated in Fig. 1 the tubular portion 24 of equipotential member 20 extends to a point adjacent the lower portion of anode tube or shell I4. In the arrangement illustrated in Fig. 2, however, in order to provide ionizing surfaces adjacent a greater area of the active surfaces I9 of the anode, the tubular portion 3| of equipotential member is of sufficient length to project within the anode tube l4 and is preferably of such length as to extend to a point within the tube I4 adjacent the extreme upper portion thereof. The tubular portion 3| is perforated to allow the arc discharge current to extend to all of the active surface of the anode.

In operation of the rectifier or inverter ID, the metal coated insulating member 20 of Fig. 1 and the corresponding member 30 of Fig. 2, while preventing the occurrence of cathode spots and consequent are back, will acquire any potential applied at any point of their surfaces as effectually as a like member formed entirely of metal. Further, the equipotential members 20 and 30 are immune to damage by are backs under all conditions of arc drop and of vacuum since the current necessary to form a cathode spot or an are on these members cannot be supplied because of the high resistance of the surface layer.

I have found that in certain instances the equipotential members 29 and 38 may be composed of carborundum. This latter material is not an extremely highly insulating material such as soapstone. It is, however, characterized, in addition to suitable mechanical strength and suitable heat-resistant properties, by a relatively high resistivity such that while an arc discharge cannot produce a destructive flow of current through the mass of the member, or a flow of current at its surface in sufficient amount to cause the formationof cathode spots thereon, the material is, nevertheless, sufficiently conductive to permit the impressing of uniform desired potentials on its surface. In case carborundum is employed, as above described, as material for the equipotential member, the applied metallic surface layer or film may be dispensed with since the carborundum itself incorporates the required high resistance surface condition.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an electric discharge device including an evacuated container, a cathode, an anode shield, an insulating and sealing cover plate for said shield, and an external anode mounted on and extending above said plate, an equipotential member comprising a relatively thick tubular portion supported by and extending through said plate, said portion having its upper end adjacent the open end of the anode and extending downwardly a short distance below the top of said plate, the inside surface of said tubular portion and its outside surface for a substantial distance below the anode being covered by a continuous thin metallic coating, said member further comprising a thick plate portion of slightly less diameter than said shield secured to the lower extremity of said tubular portion and having a plurality of openings therethrough for the passage of the arc, the surfaces of said plate portion exposed to said are having a continuous thin metallic coating in electrical connection with said first-named metallic coating.

2. In an electric discharge device including an evacuated container, a cathode, an anode shield, an insulating and sealing cover plate for said shield, and an external hollow anode mounted on and extending above said plate, an equipotential member mounted in the arc path between the cathode and anode and comprising a relatively thick tubular portion supported by and extending through said plate, said portion extending entirely within said hollow anode and having a plurality of perforations for the passage of the arc, said tubular portion extending downwardly a short distance below the top of the plate, said member further comprising a thick perforated plate portion of slightly less diameter than said shield secured to said tubular portion, the deionizing surface presented by said member extending continuously over all of said member exposed to the arc and on the outside surface of said tubular portion toa point below said anode,

said deionizing surface being sufficiently conductive to permit the charging thereof to a predetermined potential while being sufficiently resistive to prevent a'substantial flow of current thereover.

3. In an electric discharge device including an evacuated container, a cathode, an anode shield, an insulating and sealing cover plate for said shield, and an anode mounted on and extending above said plate, an equipotential member mounted in the arc path between the cathode and anode, the surface of said member being sufiiciently conductive to permit the charging thereof to a predetermined potential and sulficiently resistive to prevent a substantial flow of current thereover, said member comprising a relatively thick tubular portion supported by and extending upwardly through said plate to a point adjacent active portions of the anode and a funnel-shaped portion formed integrally with said tubular portion and projecting downwardly therefrom a short distance below the top of said shield, said member further comprising a thick circular plate secured to the larger end of said funnel-shaped portion extending substantially across said shield, said plate having a plurality of perforations therethrough for the passage of the arc.

CAMIL A. SABBAH. 

